US President Barack Obama calls for calm after George Zimmerman's acquittal over death of Trayvon Martin

US PRESIDENT Barack Obama has appealed to Americans for restraint amid anger from civil rights activists and public protests against the acquittal of a man who gunned down an unarmed black teenager.

A Florida jury comprising six women - reportedly five white and a Hispanic - late on Saturday found neighbourhood watch volunteer George Zimmerman not guilty of murdering Trayvon Martin, after a long and racially charged trial that transfixed much of the US for weeks.

Zimmerman, 29, was charged with second-degree murder, having pursued Martin, 17, through a gated community in the town of Sanford, eventually shooting him during an altercation on the rainy night of February 26, 2012.

The trial aroused strong passions and divided those who believed that Zimmerman - whose father is white and mother is Peruvian - had racially-profiled Martin, and those who believed he acted in self-defence.

Spontaneous protests broke out in San Francisco, Philadelphia, Chicago, Washington and Atlanta overnight, following the verdict, though they were mostly peaceful.

On Sunday, a large demonstration in New York attracted several thousand people, with placards that read, "Jail racist killers, not black youth'', and "We are all Trayvon. The whole damn system is guilty.''

One of the marchers in lower Manhattan wore a T-shirt proclaiming: "I'm black. Please don't shoot?''

However, Obama, the first black US president, urged people to accept the trial verdict.

"We are a nation of laws, and a jury has spoken,'' Obama said in a statement. "I now ask every American to respect the call for calm reflection from two parents who lost their young son.''

Earlier in Oakland, California, protesters had smashed windows and spray-painted cars after the verdict was announced live on television.

Obama had spoken sombrely on Martin before, noting that if he had a son he would "look like Trayvon''.

On Sunday, the President tied the killing of the teenager to the problems surrounding gun use in the United States - he had tried but failed to push through new control measures in the US congress earlier this year.

"We should ask ourselves, as individuals and as a society, how we can prevent future tragedies like this. As citizens, that's a job for all of us. That's the way to honour Trayvon Martin,'' Obama said.

"Obviously, we are ecstatic. George Zimmerman was never guilty of anything except protecting himself in self-defence,'' said his lead lawyer, Mark O'Mara, after the verdict.

Fearing violence after the Zimmerman verdict, police were out in force in Sanford, and the crowd of several hundred outside the courthouse was loud at times, but not violent.

A racial divide, however, was evident in Sanford pastor Valerie Houston's sermon on Sunday.

"Dr (Martin Luther) King (Jr) stated, the daily life of the Negro is still in the basement of the Great Society,'' she said. "And today I state, the daily life of my people is still enslaved to a white supremacist society.''

Martin's parents - father Tracy and mother Sybrina Fulton - had before the trial asked the public to respect the outcome and afterward gave thanks for the outpouring of support they received over the past year.

The Martin family's lawyer, Benjamin Crump, declined to say whether they would file a civil lawsuit against Zimmerman, but said ``they are going to certainly look at that as an option''.

"They deeply want a sense of justice. They deeply don't want their son's death to be in vain,'' he told ABC News's This Week.

Community leaders, meanwhile, called for non-violent demonstrations."There will be protests, but they must be carried out with dignity and discipline and let no act discredit the legacy of Trayvon Martin on the appeal of his family,'' civil rights leader Reverend Jesse Jackson said on CNN.

The NAACP, the largest US civil rights group, urged supporters to sign a letter asking Attorney General Eric Holder to file civil rights charges against Zimmerman.

"The most fundamental of civil rights - the right to life - was violated the night George Zimmerman stalked and then took the life of Trayvon Martin,'' read the NAACP letter.

The Department of Justice said on Sunday it continued to have an open investigation into the case, following the Florida trial.

"Experienced federal prosecutors will determine whether the evidence reveals a prosecutable violation of any of the limited federal criminal civil rights statutes within our jurisdiction,'' it said in a statement.

Most protests were peaceful.

"While the verdict may be legal, a system that doesn't take into account what happened is a broken legal system,'' said Jennifer Lue, 24, an Asian-American resident of New York.

"Everyone should feel about this, whether you're Asian-American or African-American,'' she said.

The trial has aroused strong passions among those who believed that Zimmerman - a volunteer neighbourhood watchman whose father is white and whose mother is Peruvian - racially profiled and stalked Martin, and those convinced he acted in self-defence.

The racially charged trial has transfixed the country.

Spontaneous protests broke out overnight in cities including San Francisco, Philadelphia, Chicago, Washington and Atlanta, with larger organised gatherings planned later on Sunday.

In Oakland, California, protesters smashed windows and spray painted cars, but most demonstrations were peaceful - and closely watched by police.

Zimmerman, 29, had been accused of pursuing Martin, 17, through a gated community in Sanford, Florida, and shooting him during an altercation on the rainy night of February 26, 2012.

Florida police initially declined to press charges against Zimmerman, sparking mass protests in several US cities. He was eventually arrested in April 2012 and charged with second-degree murder and manslaughter.

The spectre of the deadly April 1992 riots in Los Angeles, which broke out after a similarly racially-charged case, still lingers among law enforcement officials.

Fearing violence after the verdict, activists and community leaders appealed for calm. Police were out in force in Sanford, and the crowd of several hundred outside the courthouse was loud at times, but peaceful.

"Obviously, we are ecstatic with the results. George Zimmerman was never guilty of anything except protecting himself in self-defence," said his lead lawyer Mark O'Mara after the verdict.

Defence lawyers insist Zimmerman feared for his life after Martin attacked him, pinned him to the ground and started slamming his head against the pavement.

Zimmerman is the only living witness to how the fight began.

"Even though I am broken-hearted my faith is unshattered I WILL ALWAYS LOVE MY BABY TRAY," Martin's father Tracy wrote on Twitter.

The Martin family's lawyer Benjamin Crump declined to say whether they would file a civil lawsuit against Zimmerman.